A fuel cell is an energy conversion device that consists essentially of two electrodes, an anode and a cathode, and an electrolyte that is interposed between the anode and cathode. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or methanol, to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cells employing methanol as the fuel supplied to the anode, the electrochemical reactions are essentially as follows: first, a methanol molecule's carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule's oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air supplied to the cathode is reduced to anions with the addition of electrons. The ions formed at the anode and the cathode migrate through the interposing electrolyte and combine to form water. Thus, the electrochemical reactions of a direct methanol fuel cell (DMFC) are as follows:
Anode:CH3OH + H2O → H+ + e− + CO2E0 = 0.04V vs. NHE (1)Cathode:O2 + H+ + e− → H2OE0 = 1.23V vs. NHE (2)Net:CH3OH + O2 → H2O + CO2E0 = 1.24V vs. NHE (3)
With respect to state-of-the-art fuels cells, electrode assemblies, and electrodes, several different configurations and structures have been contemplated. For example, numerous attempts have been made to construct fuel cells and electrode assemblies that utilize a solid polymer electrolyte (SPE) as an integral part of the electrode assembly (hence, the term membrane electrode assembly (MEA) has been coined). A significant problem, however, with DMFCs utilizing solid polymer electrolytes is a phenomenon known as “methanol crossover.” As is depicted in FIG. 1, methanol in conventional DMFCs has a tendency to cross-over from the anode to the cathode via diffusion (i.e., it migrates through the electrolyte), where it adsorbs onto the cathode catalyst and reacts with oxygen from the air resulting in a parasitic loss of methanol fuel and concomitant reduction in fuel cell voltage. Indeed, performance losses of 40-100 mV at a given current density have been observed at the cathode of DMFCs utilizing a direct methanol feed (Potje-Kamloth et al., Abstract No. 105, Extended Abstracts 92-2, “Fall Meeting of the Electrochemical Society” (1992), (Kuver et al., J Power Sources 52:77 (1994)).
Exemplary solid polymer electrolyte DMFCs include those that have recently been developed by NASA's Jet Propulsion Laboratory (JPL). A detailed description of such JPL fuel cell designs may be found, for example, in U.S. Pat. No. 5,523,177 to Kosek et al., U.S. Pat. No. 5,599,638 to Surampundi et al., U.S. Pat. No. 5,773,162 to Surampundi et al., and U.S. Pat. No. 5,945,231 to Narayanan et al. Although the teachings associated with these patents have arguably advanced the art, the various membrane electrode assemblies (MEAs) disclosed therein do not eliminate the problem of methanol cross-over.
Other attempts for reducing methanol cross-over in solid polymer electrolyte DMFCs include structural modifications of the central solid polymer membrane. Exemplary in this regard are the MEAs disclosed in U.S. Pat. No. 4,664,761 to Zupancic et al., (discloses proton-conducting membrane made of an interpenetrating polymer network), U.S. Pat. No. 5,672,438 to Banarjee et al. (discloses proton-conducting laminated membrane), and U.S. Pat. No. 5,919,583 to Grot et al. (discloses proton-conducting membrane that includes an inorganic filler). Although the various MEA designs disclosed in these patents are able to reduce methanol cross-over to some degree, they nevertheless still have relatively high methanol permeabilities.
In addition to methanol cross-over, another significant problem with state-of-the-art fuel cell designs (especially solid polymer electrolyte DMFC designs) is catalytic inefficiency. For example, conventional solid polymer electrolyte DMFC designs generally attempt to maximize the surface contact between the catalyst and the solid polymer electrolyte. In this regard, it is reportedly crucial to maximize the three-phase interface that exists between the catalyst, the solid polymer electrolyte membrane, and the reactants (that permeate through the solid polymer electrolyte); such a three-phase boundary is reportedly needed to enhance efficiency and electrical capacity. As a result, a primary objective of previous DMFC research has been to optimize catalyst use by maximizing the surface area of catalyst in contact with the solid polymer electrolyte (catalyst not in direct contact with the solid polymer electrolyte has been termed “non-reacting” catalyst).
Thus, conventional methods for fabricating high-surface-area electro-catalytic electrodes for use with solid polymer electrolyte DMFCs generally include: (1) depositing on the surface of a solid polymer electrolyte either a porous metal film, a planar distributions of metal particles, or carbon supported catalyst powders; (2) embedding metal grids or meshes into the surface of a solid polymer electrolyte; or (3) embedding catalytically active components into the surface of a solid polymer electrolyte. All of these conventional methods employ traditional electrocatalyst deposition techniques such as, for example, electroplating, sputtering and metal evaporation. As such, these methods generally result in catalyst loadings in excess of 0.4 mg/cm2. A conventional state-of-the-art electrode assembly is shown in FIG. 2A, and a conventional catalyst utilization scheme is shown in FIG. 2B (wherein the three-phase interface between the catalyst, the membrane, and the reactants are shown). As shown in FIG. 2A, an exemplary conventional state-of-the-art electrode assembly 200 consists essentially of a graphite block 202 (that functions as a current collector and as a flow field), an interposing Teflon mask 204, a porous anode 206, a catalyzed membrane 208 (with embedded catalyst particles), a porous cathode 210, a second interposing Teflon mask 212, and a graphite block 214, all of which are sandwiched together. The conventional fabrication techniques and materials associated with making such state-of-the art fuel cells are not generally amenable to miniaturization or mass production.
Although significant progress has been made with respect to these and other fuel cell problems, there is still a need in the art for improved fuels cells, electrode assemblies, and electrodes. The present invention fulfills these needs and provides for further related advantages.